A Study of an Upper Layer of the Martian Ionosphere Using the Mars Express Ionospheric Sounder

Since its deployment in June of 2005, the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard ESA's Mars Express spacecraft has sent back nearly a year and a half worth of measurements of the Martian ionosphere. These data are most commonly analyzed in a format known as an ionogram, which plots the time delay of the received radar echo as a function of frequency, color coded for amplitude. An ionospheric radar echo appears as a trace exhibiting a smooth increase in time delay with frequency and an intensity typically at least two orders of magnitude higher than the background noise. As frequency increases, this trace typically displays an abrupt increase in time delay at some frequency, forming a discontinuity in the trace that we call a "cusp." These cusps indicate locations of maximums in the electron density as a function of altitude. Previous analysis of MARSIS data has shown that the main Martian ionospheric layer has a cusp corresponding to a peak electron density of roughly 105 cm-3 at an altitude of about 130 km, consistent with the results from the Viking landers in the 1970s showing that this main layer is primarily composed of O2+ and CO2+ ions. However, MARSIS ionograms have also commonly shown another cusp at a higher altitude, indicating the presence of another distinct layer higher in the ionosphere. Early analysis indicates that the peak density of this layer is typically around 104 cm-3 at an altitude of above 200 km. We believe this layer is most likely due to a peak in the concentration of O+ ions, which was previously detected at around 225 km by the Viking landers. This detection has been made at many locations in the planet's ionosphere at various spacecraft altitudes, and has shown no clear indication of a time dependence. Therefore, the presence of this layer is believed to be a normal part of the Martian ionosphere and not due to transient variations in the solar UV flux. Research is underway to more precisely analyze the properties of this layer and to confirm that this layer consists of atomic oxygen ions.